An excellent end to an excellent trilogy.
Gameplay: The same core gameplay from the first and second games is here. You have a shield and when that shield depletes you can take a few bullets until you're dead. It's been seen in many other shooters after halo(not quite sure if there were any before). Now the new gameplay element in Halo 3 is the inclusion of equipment. These are little things you can pick up and use to help you.(for example a bubble shield that you can put on the ground.) This is a great addition to the game as it can be used strategically in multiplayer.
Graphics: The graphics in halo 3 are alright, but that's all they ever get to be, alright. The fire effects look a bit, how should i say this, flat. The spartans and elites look like HD versions of their halo 2 counterparts. Though, they do have good ragdoll physics. The graphics in halo 3 are about as good as an average next-gen game, but compared to other games that came out at its time,(Cod4) they aren't that great.
Sound: One thing I always know is that bungie never lets down on the sound. From the gun sounds to the voice acting, it is all remarkable. All the right music comes at the right times and you never feel like its out of place.
Multiplayer: This is the reason you bought Halo 3. This is the reason you're going to be up at 1 in the morning screaming in the mic(hopefully you aren't one of those people) at the guy who killed you. Now I never played Halo 2 multiplayer online. Or Halo:CE multiplayer for that matter, so i wouldn't be able to compare it to those. Anyways, the multiplayer in this game is outstanding. From every weapon placement to the spawn points, it looks like it was all done with effort. The matchmaking process is quick and easy to use and you can get a game going within 3 minutes. Theres a ranking system that ranks you up as you win matches. Note: i said matches.
This, in my opinion is my only gripe about the multiplayer. The ranking system only gives you experience if you WIN the match.
Now what if you get a game where its 44-50? You were that close to winning, but the other team won. What do you get? Nothing. Absolutely nothing. All you get is some kills for your k/d ratio and another game on your online record. This can lead to some frustrating yelling at the tv in the after-game lobby, but it could also motivate you to play better. Depends on the person.
Single Player: The single-player is of average length. All the same as the other games. You can even play with 4 player co-op competing for a high score over xbox live or all on the couch. Cool huh? The story is alright for what its for. Gives you some motivation to kill the brutes, but isn't an immersive story like a game like Bioshock.
Overall: Halo 3 is a must own for any FPS fan.